For example, in the medical field, a CT device that performs tomography by using X-ray projection data, and an X-ray FPD device that monitors an X-ray projection image as a moving image are utilized, and pixel increase (data volume increase) and acceleration of various kinds of arithmetic processing are in progress in these devices (see Patent Document 1, for example).
Generally, in an X-ray detection unit of a CT device, used is an array of pixels in each of which a scintillator to generate fluorescence (scintillation light) in response to incidence of X-rays is incorporated with a photodiode (PD) that performs photoelectric conversion in response to the light (scintillation light in this case).
In each pixel, the scintillation light generated in response to the X-ray irradiation is converted to electric charge by the PD, and the electric charge is converted to a voltage value via an amplifier, an integrating circuit, and the like, and the voltage value is converted to a digital value by an AD converter of a data acquiring system (DAS) externally attached via a multiplexer, and the digital value is detected as an X-ray irradiation amount.
Note that one pixel in the above-mentioned CT device has a size of a rectangle having one side of about 0.3 to 1 mm, and a single PD is normally formed in each pixel. Since current generated by photoelectric conversion is extremely weak relative to parasitic capacitance of the PD, a current integration process of about 1 millisecond is required to detect the current.
Furthermore, acceleration of examination using the CT device is expected, and a time to photograph a subject while making one revolution around the subject is to be 0.5 seconds or less. To keep an appropriate spatial resolution at such a speed, it is necessary to increase a sampling rate of the X-ray detection unit, and it is necessary to shorten a detection period. For example, in a case where each pixel performs 1000 times of sampling during a revolution period of 0.5 seconds, the detection period is needed to be shortened to 0.5 msec.